It is well-known to provide an automatic control to upshift and downshift a transmission in accordance with predetermined shift schedules wherein the upshift and downshift points for each speed ratio varies somewhat in response to speed and power setting signals of an engine. The speed signal is often proportional to the transmission output speed and the power setting signal is often directly or indirectly proportional to throttle or rack position.
Such normal or automatic shift schedules are designed for the most often encountered or average driving needs. In low power-to-weight vehicles, such as trucks, the operator often prefers a somewhat modified or inhibited shift schedule when high performance is needed. To facilitate modifying or inhibiting the normal shift schedules, control systems are often provided with manual shift selectors having, in addition to the drive or normal automatic shift schedule position, one or more inhibit positions. For example, the shift selector in a truck with a four-speed transmission may have third, second, and first speed ratio positions which inhibit or raise the normal shift points of all speed ratios above the selector position.
Prior art hydraulic control systems having the mentioned shift inhibit feature often have a shift valve valving member spring biased to a downshift position and biased to an upshift position by governor pressure proportional to the transmission output speed. The valving members are also often biased or acted upon by a modulator pressure proportional to the power setting of the engine. The modular pressure may be directly or inversely proportional to the power setting and most often functions to raise the upshift point of the valving member with increasing power settings. The prior art shift inhibitors in such hydraulic control systems traditionally direct a rather precisely controlled inhibit pressure to the valving member. The inhibit pressure overrides the spring and biases the valving member toward the downshift position with a force sufficient to raise the upshift and downshift points but insufficient to raise the shift points to a level which would cause engine damage.